1. Field of the Invention
The present invention relates to a flat panel display, and more particularly, to an organic EL device and a method for fabricating the same.
2. Discussion of the Related Art
In recent years, as the size of the display devices increases, demand on a flat panel display occupying a small space increases. As one of the flat panel displays, an organic electro-luminance (EL) device called an organic light emitting diode (OLED) is being developed in a rapid speed, and various prototypes have been published.
The organic EL device is a device to emit light while an electron and a hole are injected into an organic emission layer disposed between a first electrode, electron injection electrode (cathode) and a second electrode, hole injection electrode (anode), the electron and the hole are bonded to form a pair of electron and hole and generate an exciton, and the generated exciton disappears from an excited state to a base state.
Such an organic EL device is being actively researched due to a relatively low operation voltage of 5-10 V compared with a plasma display panel (PDP) or an inorganic EL display.
Also, since the organic EL device has superior features such as a wide viewing angle, a high speed response and a high contrast, it can be used as a pixel of a graphic display, a television image display or a pixel of a surface light source. Further, since the organic EL device can be formed on a flexible transparent substrate and has slim, lightweight and good color sense characteristics, it is suitable for a next generation flat panel display (FDP).
Furthermore, since the organic EL device does not need a backlight compared with the LCD well known, it has advantages such as a low power consumption and a superior color sense.
In general, the organic EL devices can be classified into a passive type and an active type.
First, a passive type organic EL device will be described.
FIGS. 1A and 1B are plane views illustrating a process of fabricating a passive type organic EL device according to the related art.
In the process of fabricating a passive type organic EL device, an indium tin oxide (ITO) strip 2 serving as anode is formed on a glass substrate 1 as shown in FIG. 1.
At this time, a short ITO strip 2-1 is formed at a portion where one end of cathode is being formed, simultaneously with the formation of the ITO strip 2. Forming the short ITO strip 2-1 in advance is to make it easy to form a metal extension line to be connected with cathode.
If necessary, an auxiliary electrode is formed on the ITO strips 2 and 2-1, and then an insulating layer 3 is formed on the auxiliary electrode. Next, for insulation between cathodes, an electrical insulation separator 4 is formed.
Next, as shown in FIG. 1B, an organic EL layer 5 including a hole transporting layer, an emission layer and an electron transporting layer is coated and then a cathode 6 is formed of a conductive material such as an Mg—Ag alloy, aluminum (Al).
At this time, the cathode 6 can contact the short ITO strip 2-1 by not forming the organic EL layer 5 on the short ITO strip 2-1 but forming the cathode 6 on the short ITO strip 2-1.
Lastly, a seal plate 8 is attached on a resultant structure by a sealant 7, thereby completing the passive type organic EL device.
Next, an active type organic EL device will be described with reference to the accompanying drawings.
FIGS. 2A and 2B are plane views illustrating a process of fabricating an active type organic EL device according to the related art.
First, as shown in FIG. 2A, an active area 12 and a pad 13 are formed on a glass substrate 11.
Thereafter, a metal line 14 electrically connected with the pad is formed at a portion where one end of a cathode is positioned so as to make it easy to connect the metal line 14 with the cathode.
Although not shown in the drawings, the active area 12 includes a plurality of unit cells arranged in a matrix configuration, each unit cell including a drive transistor (TFT) and an anode connected to drain electrode of the drive TFT.
Continuously, as shown in FIG. 2B, an organic EL layer 15 is formed on the active area 12.
At this time, the organic EL layer 15 should not be formed on the metal line 14 so that the metal line 14 may contact the cathode to be formed later.
Next, a cathode 16 is formed on the organic EL layer 15. At this time, the cathode 16 is formed extending to an upper surface of the metal line 14 such that the cathode 16 can contact the metal line 14.
Next, a passivation layer (not shown) including an oxygen adsorbent layer, a moisture absorbent layer and a moisture permeation preventing layer is formed on a resultant structure and then a seal plate 18 is attached on the passivation layer using a sealant 17, thereby completing the active type organic EL display panel.
The above organic EL device is required to form first the organic EL layer and secondly the cathode metal film in a vacuum state regardless of whether the organic EL device is the passive type or the active type.
The metal thin film can be formed by one of three deposition methods consisting of a vacuum evaporation, a sputtering and an ion-plating.
Among the three methods, the vacuum evaporation is the most widely used to form the cathode.
Also, the vacuum evaporation can be further classified into four types consisting of a resistance heating evaporation, an E-beam heating evaporation, a RF heating evaporation, and a laser beam heating evaporation.
Among the four evaporation methods, the resistance heating evaporation is the most widely used to form the cathode.
FIG. 3 is a schematic view illustrating a resistance heating evaporation process.
In order to form the cathode of a thin metal film using the resistance heating evaporation, a target material to be evaporated, for example, a boat 31 filled with aluminum (Al) is disposed at a lower side of a chamber.
Next, a substrate 32 is positioned at a sample holder disposed at an upper side of the chamber, and then a mask 33 is aligned below the substrate 32 such that a thin film having a desired pattern corresponding to a pattern of the mask 33 is formed on the substrate 32.
Next, the chamber is made in a vacuum state and the boat 31 is heated by applying a current thereto, so that the Al received in the boat 31 is evaporated.
The evaporated Al molecules are deposited on the substrate 32 to form a cathode 34.
However, the above resistance heating evaporation process has disadvantages such as many preheating time and frequent exchange of the boat due to limitation in capacity of the boat receiving the target material.
Also, it is required to release the vacuum state of the chamber whenever the boat is exchanged. To make the exchanged boat be in a vacuum state, 2-3 hours are wasted, which is unsuitable for mass production.
Accordingly, the E-beam heating evaporation process having a faster preheating time than in the resistance heating evaporation process and using a crucible instead of the boat is presently employed in the mass production.
FIG. 4 is a schematic view illustrating an E-beam heating evaporation process.
To form a cathode of a thin metal film using the E-beam heating evaporation process, a target material to be evaporated, for example, a crucible 41 filled with aluminum (Al) is loaded at a lower side of a chamber.
Next, a substrate 42 is positioned at a sample holder disposed at an upper side of the chamber, and then a mask 43 is aligned below the substrate 42 such that a thin film having a desired pattern corresponding to a pattern of the mask 43 is formed on the substrate 42.
In addition, a filament 44 emitting electrons is positioned at a side portion of the crucible 41, and a magnet 45 is also positioned around the crucible 41 so as to provide a magnetic force for transferring the electrons emitted from the filament 44 to the crucible 41.
Thereafter, as the filament 44 is heated, electrons are emitted therefrom. The emitted electrons are curved in the form of a circle by the magnetic force of the magnet 45 and collide with a center of the Al target material.
Accordingly, the Al target material is heated up to an evaporation temperature due to the electrons having a high energy and is evaporated. The evaporated Al molecules are deposited on the substrate 42 to form the cathode 46.
However, forming the cathode 46 using the related art E-beam heating evaporation has the following problem.
Charges are accumulated on the cathode by the electrons emitted from the filament 44, so that the performance of the organic EL layer and the TFT is lowered, resulting in a low reliability.